2,7-DMN is oxidized to produce naphthalene 2,7-dicarboxylic acid, which is an industrially important material for manufacturing polyesters. 2,7-DMN is contained in various fractions of petroleum or coal tar as mixtures together with other DMN isomers.
There have hitherto been proposed several methods for separating 2,7-DMN from these fractions.
For example, one of the proposed methods involves concentrating and extracting a DMN fraction from a raw material, such as petroleum or coal tar, using suitable techniques, such as fractionation; cooling the DMN; concentrate to remove, i.e. crystalize out, a portion of 2,6-DMN; treating the remaining DMN mixture which contains the 2,6- and 2,7- isomers in the ratio of their eutectic mixture with a zeolite adsorbent to selectively adsorb the 2,7-DMN; and isolating the 2,7-DMN by subjecting the adsorbed material to a desorption treatment. The conventional crystallization method, however, does not yield 2,7-DMN having a sufficiently significantly high purity because, although the DMN mixture is rich in 2,7-DMN, it still contains 2,6-DMN having sufficiently high melting point, and thus the crystallization method only yields a eutectic mixture.
As described above, various fractions of petroleum or coal tar contain substantially equivalent amounts of 2,7-DMN and 2,6-DMN and many additional components such as, for example, the other DMN isomers.
2,7-DMN and 2,6-DMN have melting points which are very close to each other. Thus, even with conventional separation techniques, these isomers tend to form eutectic mixtures for solid solutions.
There are other problems associated with the conventional techniques for separating 2,7-DMN from a mixture of dimethylnaphthalenes. For example, there are generally lower recoveries of 2,7-DMN than are desired. In addition, the purity of the 2,7-DMN which is isolated, is somewhat lower than desired, and the purity can be increased only with great difficulty and at excessively high costs through further or alternative separation and purification procedures.